Focused ion beam (FIB) systems are used in a variety of applications in integrated circuit manufacturing and nanotechnology to create and alter microscopic and nanoscopic structures. FIB systems can use a variety of sources to produce ions, such as a plasma source or a liquid metal ion source (LMIS), which are then focused onto a sample for imaging and milling. In recent years, various FIB systems utilizing a plasma source to generate a plasma focused ion beams (P-FIBs) have been developed. P-FIB systems are capable of producing higher current focused ion beams than may typically be achieved by FIB systems utilizing other ion sources, such as liquid metal ion sources. The higher current focused ion beams of PFIB systems allow faster milling rates to be achieved in sample-processing applications, which may be particularly desirable, for example, in high-throughput sample-processing applications.
One issue encountered when using FIB systems, particularly P-FIB systems, is staining around areas of a sample irradiated with the plasma-focused ion beam. For example, a large circular stain, or “halo,” may form around an impact point of the plasma-focused ion beam on the surface of a sample. It is believed that staining from plasma-focused ion beams arises from neutral particles (“neutrals) striking the sample and implanting into the sample material, or inducing deposition of contaminants, either directly or indirectly through generation of secondary electrons (SEs). The neutrals may be neutral particles leaked from the ion source of the FIB column and/or ions of focused ion beams that are neutralized in transit from the ion source to the sample. Compared to FIB devices utilizing LMIS sources, staining is significantly more pronounced in P-FIB systems because plasma sources operate at gas pressures orders of magnitude higher than a typical LMIS, and since P-FIBs are usually used to generate larger beam currents, the vacuum conductance of the larger beam-defining apertures (BDAs) allows more gas to leak down the column, eventually reaching the sample.
Thus, there is a need for new ways of preventing neutrals from reaching samples processed by single beam FIB columns. In particular, it would be desirable to find new apparatuses and methods for blocking neutrals generated by P-FIB systems from reaching samples processed by plasma focused ion beams generated therefrom to reduce or eliminate sample staining.
Image saturation is another issue encountered when P-FIB devices are used in dual beam systems. In dual-beam systems, comprising both a P-FIB column and a SEM column having optical axes intersecting at a common target, a typical work flow may constitute alternating P-FIB-milling steps with SEM imaging steps. During milling, the SEM beam is typically blanked while the P-FIB removes material by irradiating the sample. The P-FIB beam is then typically blanked, and the SEM beam is used to image the sample for end-pointing and other process-control functions. What has been discovered is that the plasma source, and also the P-FIB column, produce an appreciable flow of neutrals towards the sample, which cannot be blanked, deflected, or focused by the P-FIB column. When this neutral current strikes the sample during SEM imaging, it can induce secondary electron emission which may leave the sample and be detected by the imaging detector, causing image saturation. In some cases, particularly when the PFIB column has been configured to produce large ion currents, the neutral current may generate such a large background signal that “white-out” conditions are created, making SEM imaging is impossible.
Thus, there is a need for new ways of preventing neutrals from a FIB column of a dual beam FIB/SEM system from reaching samples during P-FIB milling, and also during SEM imaging while blanking the plasma-focused ion beam. In particular, it would be desirable to find new apparatuses and methods for blocking neutrals generated by P-FIB columns of dual beam P-FIB/SEM systems to reduce or eliminate staining when processing samples with plasma focused ion beams, and to reduce or eliminate image saturation when imaging samples with the SEM while blanking the P-FIB column.